Production of protective coatings on aluminum and alloys thereof



Patented Jan. 10, 1950 raonoo'rlon or" PROTECTIVE COATINGS ,oN ALUMINUM AND ALLOYS THEREOF 1 Claude L. Waring, Glen Cove, N. Y., assignor t0 Reynolds Metals C0mpany,'Richmond, Va., a

corporation of Delaware Application December 27, 1946, Serial No. 718,90 i

' I v No Drawing.

' 2 Claims. 1

The object of the present invention is to provide a surface for aluminum and aluminum base alloys which will have a very high corrosion resistance along with a pleasing gray coloration. The method is characterized by the employment of chemical reaction between the aluminum or the aluminum alloy surface and a solution of a special chemically active combination of substances. In one phase of the invention an iridescent coating is obtained.

A characteristic of the invention is the employment of two materials heretofore known in the production of oxide coatings for aluminum or aluminum alloys, to wit, sodium carbonate and potassium dichromate in the presence of two additional materials, thereby forming a combination which results in a markedly superior coating, both as to corrosion resistance and appearance, and which, when one of the materials, potassium dichromate, is closely controlled as to amount, results in an iridescent coating.

In the practice of my method the aluminum or aluminum alloy article, whether sheet or other form, is first freed from grease and other surface impurities by treatment with a mild alkaline cleaner or by means of a vapor de-greasing agent and is then immersed in a hot bath containing the following constituents, the bath being aqueous:

Per cent by weight Sodium carbonate 5- Sodium hydroxide .l- .4 Potassium dichromate .2- 2 Aluminum chloride .1- .3

The bath is preferably maintained at a temperature of 180-212 F., and the time of treatment is between 5 and minutes, depending upon the desired shade to be obtained in the final coating, the coating increasing in depth of coloration with increase in the time of treatment from the stated minimum time.

After the treatment has been effected, the coated article is removed from the bath and thoroughly washed, which washing may be effected in cold water. It is desirable at this point to neutralize any trapped alkali in the coating. This result may be obtained, together with improved sealing of the coating, by the immersion of the aluminum or aluminum alloy article in a second aqueous bath of chromic acid. This bath may be relatively weak, and an operative range of chromic acid for the purpose is .012 %-.1%, the solution preferably being employed within a temperature range of 135 F.-145 F., for a time of 2 5-10 minutes. The article may then be removed from the chromic acid bath and thoroughly Washed in hot water, a Washing period of five minutes being entirely sufficient.

With the oxidizing bath above specified, and the time range of treatment, the bath will re-act with and remove a layer of 0001-0005 thickness from the surfaces of usual types of aluminum alloys, while the constituents of the alloy with the chemicals of the bath will form oxides of aluminum, chromium, etc., depending upon the composition of the alloy, in the pores thereof and on the surface to the extent of 0001-.0005" thickness.

For the production of an iridescent coating the potassium dichromate is employed in the proportion of about .5% in the bath, and the time of treatment in the bath should be approximately 3 minutes.

Tests of aluminum alloys treated in accordance with the above method have shown superior resistance to corrosion, and better appearance, as compared with usual oxide coatings, including those produced by sodium carbonate and potassium dichromate mixtures. Aluminum alloy sheet protected by the coating is excellent in the manufacture of many articles subjected to air and moisture, and particularly roofing, sheeting and other metallic building elements, and the surfaced metallic elements thus produced require no painting for pleasing effect, inasmuch as the coloration is substantially neutral with respect to the primary colors.

Although it has been considered in the past that chromic acid attacks oxide coatings, and hence treating solutions for such coatings on aluminum surfaces should be free from chromic acid, I have found a very definite benefit in the use of chromic acid for final treatment of my oxide coating, when the acid is used in the control range specified above, and within the specified temperature range. As stated, any alkali which remains trapped after the washing of the coating following the primary bath, is neutralized by the weak chromic acid solution and the solution has a sealing effect.

What I claim as new and desire to secure by Letters Patent, is as follows:

1. A process for treating alloys predominantly aluminum rendering said articles substantially resistant to corrosion comprising the steps of subjecting the article to an aqueous bath consisting of sodium carbonate, 5 to 10 percent; sodium hydroxide, 0.1 to 0.4 percent; potassium dichromate, 0.2 to 2.0 percent; aluminum chloride, 0.1 to 0.3

percent; said bath being maintained at a temperature of from 180 to 212 F., for a period of from to minutes, washing the so-treated article, subsequently subjecting the article to a second bath, consisting of from .012 to 0.1 percent of chromic acid in aqueous solution at temperatures ranging from to F. for a period of from 5 to 10 minutes.

2. A process for treating and aluminum alloys predominantly aluminum, to render them corrosion resistant consisting of subjecting the articles to an aqueous bath containing sodium carbonate, 5 to 10 percent; sodium hydroxide, 0.1 to 0.4 per cent; potassium dichromate, 0.2 to. 2.0 percent; I

aluminum chloride, 0.1 to 0.3 percent; said bath being maintained at a temperature of from 4 0.012 to 0.1 percent thereof in aqueous solution, for a period of from 5 to 10 minutes.

CLAUDE L. WARING.

REFERENCES CITED 7 The, f ollowing references; are of record in the to 212 F. for a period of from 5 to 20 minutes,

washing the coating, subjecting the washed coatfileof .this patentz V UNITED STATES PATENTS ing to a bath of chromic acid containing from Great Britain Mar. 5, 1925 

1. A PROCESS FOR TREATING ALLOYS PREDOMINANTLY ALUMINUM RENDERING SAID ARTICLES SUBSTANTIALLY RESISTANT TO CORROSION COMPRISING THE STEPS OF SUBJECTING THE ARTICLE TO AN AQUEOUS BATH CONSISTING OF SODIUM CARBONATE, 5 TO 10 PERCENT; SODIUM HYDROXIDE, 0.1 TO 0.4 PERCENT; POTASSIUM DICHROMATE, 0.2 TO 2.0 PERCENT: ALUMINUM CHLORIDE. 0.1 TO 0.3 PERCENT; SAID BATH BEING MAINTAINED AT A TEMPERATURE OF FROM 180 TO 212*F., FOR A PERIOD OF FROM 5 TO 20 MINUTES, WASHING THE SO-TREATED ARTICLE, SUBSEQUENTLY SUBJECTING THE ARTICLE TO A SECOND BATH, CONSISTING OF FROM .012 TO 0.1 PERCENT OF CHROMIC ACID IN AQUEOUS SOLUTION AT TEMPERATURES RANGING FROM 135 TO 145*F. FOR A PERIOD OF FROM 5 TO 10 MINUTES. 